1. Field of the Invention
The present invention relates to a light irradiation apparatus, a light irradiation method, a crystallization apparatus, and a crystallization method, and relates to, e.g., a technology of irradiating a non-single crystal semiconductor film with a laser beam having a predetermined light intensity distribution to generate a crystallized semiconductor film.
2. Description of the Related Art
A thin film transistor (TFT) used for, e.g., a switching element that selects a display pixel in a liquid crystal display (LCD) and the like is conventionally formed of amorphous silicon or poly-crystal silicon. It is known that poly-crystal silicon has a high mobility of electrons or holes than that of amorphous silicon.
Therefore, when a transistor is formed of poly-crystal silicon, a switching speed becomes higher and a display response speed also becomes higher than those in an example where a transistor is formed of amorphous silicon. Further, an LSI arranged at a peripheral portion of the device, e.g., a driver circuit or a DAC can be constituted of a thin film transistor to operate at a higher speed. Furthermore, there is also an advantage of, e.g., a reduction in design margins of other components.
Since poly-crystal silicon is made of an aggregation of crystal grains, when, switching transistor such as a TFT transistor is formed of poly-crystal silicon, crystal grain boundaries inherently present in a channel region of the transistor. These crystal grain boundaries serve as barriers, and hence a mobility of electrons or holes becomes lower than that of a TFT transistor formed of single-crystal silicon. Many thin film transistors respectively formed of poly-crystal silicon, the number of crystal grain boundaries formed in a channel region differs depending on each thin film transistor. This difference becomes unevenness, resulting in a problem of display unevenness in case of a liquid crystal display using such a transistor. Thus, in order to improve the mobility of electrons or holes and reduce unevenness in the number of crystal grain boundaries in the channel portion, a crystallization method of generating crystallized silicon or poly-crystal silicon having a large particle diameter that enables forming at least one channel region has been proposed.
As this type of crystallization method, the following technology has been conventionally proposed. That is, according to this technology, an incident laser beam is modulated into a laser beam having a V-shaped light intensity distribution that one-dimensionally varies in a predetermined direction by using a light modulation element (a phase shifter), as follows. The modulation element has a phase pattern in which an area share ratio of a phase modulation region in a unit area one-dimensionally varies in a predetermined direction. A non-single crystal semiconductor film (a poly-crystal semiconductor film or a non-single crystal semiconductor film) is irradiated with this modulated laser beam, so that the film is subjected to crystal growth in the predetermined direction, thereby generating a crystallized semiconductor film (see, e.g., KOKAI 2004-343073, and Y. Taniguchi, etc., “A Novel Phase-modulator for ELA-based Lateral Growth of Si”, The electrochemical Society's 206th Meeting, Thin Film Transistor Technologies VII (Honolulu, Hi.)).
As shown in FIG. 22A, a conventional crystallization technology proposed in the document uses a light modulation element 101 having a phase pattern in which an area share ratio of a phase modulation region in a unit region one-dimensionally varies in a predetermined direction (a horizontal direction in FIG. 22A). In this figure, each hatched square region 101a is the phase modulation region, and its area is reduced from a central part toward a peripheral part. A laser beam modulated via this light modulation element 101 has a V-shaped light intensity distribution that one-dimensionally varies on an image plane of an image forming optical system, that is an irradiated surface of the silicon film. Specifically, when a phase modulation amount of the phase modulation region 101a of the light modulation element 101 is 60 degrees, a V-shaped light intensity distribution 102 indicated by a thick solid line in FIG. 22B is theoretically generated. Further, when a phase modulation amount of the phase modulation region of the light modulation element 101 is 180 degrees, a V-shaped light intensity distribution 103 indicated by a thick solid line in FIG. 22B is theoretically generated, and a V-shaped light intensity distribution 104 indicated by a thin solid line in FIG. 22B is actually produced. When a non-single crystal semiconductor film is irradiated with a laser beam having a V-shaped light intensity distribution generated in this manner, a crystal grows in a gradient direction of the light intensity distribution, and each needle-like crystal 105 that extends in the gradient direction from a central part where a light intensity is low is generated as shown in FIG. 22C.
In a conventional crystallization technology, when a coherence factor, that is, a σ-value (an emission-side numerical aperture of an illumination system/an object-side numerical aperture of an image forming optical system) is set to a comparatively small value (e.g., 0.5 or less), a desired light intensity distribution can be generated. However, if the σ-value is set to a comparatively large value (e.g., 0.6 or less), for example, a wavelike undulation is generated on a contour line indicating a light intensity, and the desired light intensity distribution cannot be generated. As a result, unevenness is generated in the growth of the crystals, the crystal grains having a desired shape cannot be generated, and in consequence, electric properties of the manufactured TFT inconveniently fluctuate.